Operating at high-temperature and high-pressure, the thick wall of a hydroprocessing reactor is saturated with atomic hydrogen. When the reactor is cooled rapidly, the vessel wall becomes supersaturated with hydrogen; this can cause base-metal cracking and/or overlay disbonding which are forms of reactor damage. To avoid damage, the wall must be "outgassed" before cooling to the critical temperature. Historically, outgassing has been slow and may require more than one week depending on operating conditions and the geometry and properties of the vessel wall. This lengthens the turnaround time and reduces the productivity and profitability of the process operation. Conventional outgassing involves: (a) slowly lowering the reactor temperature from process operating temperature to ambient temperature and (b) gradual hydrogen depressurization only after the temperature drops to about 575.degree. K. to prevent coking the hydroprocessing catalyst contained in the reactor. Actual cooling rates and depressurization rates are largely trial and error since, during the cooldown period, the operator will have only limited knowledge of the transient hydrogen concentration profile across the reactor wall.
The problems of hydrogen blistering and overlay disbonding are well known in the refining industry. For a discussion of outgassing and hydrogen content in reactor steels, see The Effect of Outgassing Cycles on the Hydrogen Content in Petrochemical-Reactor-Vessel Steels, API Publication 946 (Jul. 1981); Hydrogen-Induced Disbonding of Stainless Weld Overlay Found in Desulfurizing Reactor, J. Watanabe, Performance of Pressure Vessels With Clad and Overlayed Stainless Steel Linings, Denver, Colo., 21-25 (June 1981), ASME, 8212-72-0542; Material Problems in the Hydrocarbon Processing Industry, G. R. Prescott, Met. Prog., 120, (2) 24-30 (July 1981); A Safety Analysis on Overlay Disbonding of Pressure Vessels for Hydrogen Service, T. Fujii, Current Solutions to Hydrogen Problems in Steels, Washington, D.C., 361-368 (November 1982), ASM, 8303-72-0154.